Real estate mogul Harry Macklowe and wife, Linda, are splitting after 57 years of marriage, Page Six has exclusively learned.

Macklowe announced to his wife last week that he was leaving her for a new girlfriend — Patricia Landeau, president of the French Friends of the Israel Museum, located in Jerusalem. Multiple sources said Harry was even gallivanting around the Hamptons over Memorial Day weekend with his attractive new French gal pal.

“Harry told his wife last week he was divorcing her — completely out of the blue — then moved straight in with his girlfriend,” a source said. “This is a total shock to everybody because he’s been married to Linda a long time. Harry is almost 80, he’s been married for [almost] 60 years, and suddenly he has a girlfriend. And he’s been running around town and the Hamptons with her.”

Another source said, “He could have handled it better . . . it’s so in Linda’s face.” (Harry’s well-known for sharing a home with Linda on Georgica Pond, where he famously feuded with neighbor Martha Stewart.)

The first source added of the Macklowes: “This is going to be a blockbuster divorce: There’s a few billion dollars at play here, plus an astonishing art collection.” Harry and Linda’s art collection — spearheaded by Guggenheim Foundation trustee Linda — is worth a reported $1 billion and includes pieces by Franz Kline, Mark Rothko and Gerhard Richter. Harry’s current real estate projects include 432 Park Ave.

He previously owned the GM Building and developed the Apple store on Fifth Avenue.

Meantime, the Macklowes are donors to the Israel Museum and regulars at its New York gala. Sources said Harry met Landeau at a museum event.

But a source said of Landeau, “It’s not like she’s so young . . . she has four kids. But she’s French, blond and very attractive.” A rep for Harry Macklowe did not comment. Landeau could not be reached.